The first lens I bought for surf shots was the Sigma 50-500 f/4, which was only about $1000 new. This worked great and gave me very good range. I agree with croasdial that you won't need 2.8 for surf shots. The lens worked well for most outdoor sports but I eventually switched to the 70-200 f/2.8L IS for indoor indoor shooting and will use a 1.4x if needed for surfing. I just got it and have yet to get it to the beach.
BTW, the 600mm is a beast to lug around, I don't think it's necessary unless you shoot reef passes from the shore. Most guys I know use the 70-200 from a boat or the back of a jet ski (very carefully).

